Biomechanics of Soft Tissue

Question 1

What is the tendon composition?

Answer 1

70% water
30% ➡️ 70-90% Type 1 Collagen
              10-15% Cells
              2-5% Proteoglycans
              0.5-5% Other Glycoproteins
              0.5-3% Elastin

Question 2

What is the structure of a tendon?

Answer 2

It is a fibre composite material
➡️ multiple hierarchical levels of collagen
➡️ Proteoglycanous matrix binding
➡️ Interspersed with cells (tenocytes)

Question 3

What can cause a change in the tissue cells?

Answer 3

Mechanical stimuli

Question 4

What is the cellular response mechanical stimuli causes in tissue cells?

Answer 4

Proliferation
Matrix synthesis
Matrix degradation
Cell/matrix orientation

Question 5

In terms of tendons, what is homeostasis?

Answer 5

Normal tissue turnover

Question 6

In terms of tendons, what is disease/degeneration?

Answer 6

Tissue degradation

Question 7

In terms of tendons, what is tissue repair?

Answer 7

Tissue anabolism and regeneration

Question 8

What are the properties of an Energy Storing tendon? (6)

Answer 8

Eg Human Achilles, Equine SDFT

• high strains
• elastic recoil in use
• high incidence of tendinopathy
• less stiff
• highly fatigue resistant
• less viscoelastic

Question 9

What are the properties of Positional tendons? (6)

Answer 9

Eg human anterior tibialis, equine CDET

• low strains
• less elastic
• efficient strain transfer from muscle to bone
• more stiff
• less fatigue resistant
• more viscoelastic

Question 10

What happens when strain is applied to energy storing tendons?

Answer 10

➡️ fibre sliding

• significantly more fibre sliding in extensor tendons
• significantly better recovery from loading in energy storing flexor fascicles
• significantly less hysteresis (better recovery) in energy storing flexor fascicles
• extension = sample rotation
• good recovery & elasticity

Question 11

What happens when strain is applied to a positional tendon?

Answer 11

➡️ rotation of fibres

• significantly more fascicle rotation in flexor tendons
• extension = fibre sliding
• poor recovery and less elastic

Question 12

What happens in aging and energy storing tendons?

Answer 12

• The tightness of the helix reduces with age
• the recoverability of the helix reduces with age
• the capacity for fascicle sliding reduces with age

Question 13

What is the function of a tendon?

Answer 13

It transmits muscle forces to the skeleton
- provides a link from compliant muscle to stiff bone
- acts as a lever arm which reduces the need for large muscles
- reduces the need to have muscles near joints
- efficiently transfers forces
▶️▶️ limited extensibility
▶️▶️ some protection from impact loading
▶️▶️ can store energy to assist in locomotion

Question 14

When is tendinopathy not tendinitis?

Answer 14

If it is chronic

Question 15

What is the pathological process of tendinopathy?

Answer 15

LOAD - causing balanced tenocyte catabolism/anabolism
⬇️overload
A REACTIVE tendon
⬇️ tenocyte mediated disease process, failed healing
A DEGENERATIVE tendon

Question 16

Tenon loading in younger people with tendinopathy

Answer 16

Reasonable strength
High load demands
Isometrics, eccentrics, concentric a
Power
Endurance➡️load➡️speed

Question 17

Tendon loading in middle age recreational athletes with tendinopathy

Answer 17

Reasonable strength
Lower load demands
Isometrics, eccentrics, concentrics
Power
Endurance➡️load➡️speed

Question 18

Tendon loading in older or sedentary patients with tendinopathy

Answer 18

Weak - intrinsic factors of adiposity or menopause
Isometrics, eccentrics, concentrics
Endurance➡️load➡️speed

Question 19

Define Hooke’s Law

Answer 19

An increase in stress causes a proportional increase in strain

Question 20

Define Youngs Modulus

Answer 20

The ratio of stress to strain

Question 21

Define the elastic range

Answer 21

The range in which something can be stretched, where when released it will return to its original shape

Question 22

Define the plastic range of something

Answer 22

The range in which as a force is applied the shape is deforming permanently

Question 23

Define poisons ratio

Answer 23

The ratio of the proportional decrease in a lateral measurement to the proportional increase in length in a sample which is elastic ally stretched

Question 24

Define generalised Hooke’s Law

Answer 24

Strain can exist without stress

Question 25

Which type of collagen is most common in connective tendons and ligaments?

Answer 25

Type 1

Question 26

How is collagen organised and why?

Answer 26

Organised structurally to allow both tensile stiffness and strength to tissue

Question 27

What is the structure of the basic unit of collagen?

Answer 27

Tropocollagen to collagen triple helix - 3 peptide alpha chains - peptide chains packed together by hydrogen bonds - tropocollagens covalently bonded to other neighbouring tropocollagens to form staggered D-periods - microfibrils➡️subfibrils➡️collagen fibrils

Question 28

What is the ligament structure? And what is the function of this structure

Answer 28

75-80% dry weight = type 1 collagen 55-65% wet weight = water Thought to provide lubrication Influences viscoelastic

Question 29

What are the function of tendons

Answer 29

- Connects muscle to bone - force transmission between the 2 structures - vary in size and shape in the body - - can pass through bony features - -specialised connective tissue sheaths guide tendon across joints to maintain correct orientation and force transduction

Question 30

What is the primary function of a tendon?

Answer 30

Transmit muscle force

Question 31

What is the difference between tendons and ligaments?

Answer 31

- Collagen fibres in tendon tend to be aligned in tendon compared to ligaments - fewer fibroblasts than in ligaments

Question 32

What are the stages in muscle contraction?

Answer 32

1. Myosin head attaches to the actin myofilament, forming a cross bridge 2. Inorganic phosphate generated in the previous contraction cycle is released, initiating the power stroke. The myosin head pivots and bends as it pulls on the actin filament, sliding it towards the M line. Then ADP is released 3. As the new ATP attaches to the myosin head, the link between myosin and actin weakens and the cross bridge detaches 4. As ATP is split into ADP and Inorganic phosphate, the myosin head is energised (cocking into the high energy confirmation)

Question 33

Define the motor unit

Answer 33

Single motor neurone and the muscle fibres it innervates | - hamstrings 300:1 muscle/nerve ratio

Question 34

What is the size principle of motor unit recruitment?

Answer 34

- Orderly hierarchy of motor unit recruitment based on size - Low levels of force; small (low force) motor unit with thin axons and small, easily excitable motor unit are recruited - As force requirements increases, systematic recruitment of larger motor unit

Question 35

What are the biochemical properties of muscle fibres?

Answer 35

Oxidative capacity | Type of ATPase

Question 36

What are the contractile properties of muscle fibres?

Answer 36

Maximal force production Speed of contraction Muscle fibre efficiency (mechanical work: metabolic cost)

Question 37

What is the difference between contraction in Type 1, type 2a and 2b fibres?

Answer 37

Type 1 contraction slow Type 2a contraction medium Type 2b contraction fast

Question 38

What is the difference between size of motor neuron in Type 1, type 2a and 2b fibres?

Answer 38

Type 1: small Type 2a: medium Type 2b: very large

Question 39

What is the difference between resistance to fatigue in Type 1, type 2a and 2b fibres?

Answer 39

Type 1: high Type 2a: fairly high Type 2b: low

Question 40

What is the difference between activity the fibre is used for in Type 1, type 2a and 2b fibres?

Answer 40

Type 1: aerobic Type 2a: long term anaerobic Type 2b: short term anaerobic

Question 41

What is the difference between maximum duration of use in Type 1, type 2a and 2b fibres?

Answer 41

Type 1: hours | Type 2a:

Question 42

What is the difference between power of contraction in Type 1, type 2a and 2b fibres?

Answer 42

Type 1: low Type 2a: medium Type 2b: very high

Question 43

What is the difference between major fuel storage in Type 1, type 2a and 2b fibres?

Answer 43

Type 1: triglycerides | Type 2a&b: creatine phosphate and glycogen

Question 44

In hills three-element muscle model, define the active force of the contractile element

Answer 44

The active force comes from the force generated by the actin and myosin crossbridges at the sarcomere level

Question 45

In hills three-element muscle model, define the three elements

Answer 45

Contractile element- Force generation from sarcomere from crossbridge formation 2 nonlinear springs- - parallel element (passive force of when endomysium when stretched) - series element (tendon and intrinsic elasticity of myofilaments)

Question 46

What happens to the force of the muscle when the sarcomeres lengthen? Why?

Answer 46

Active force decreases Passive force increases The rate of detachment of actin myosin crossbridge sits slower during sarcomere lengthening so there is a greater force

Question 47

What is the force generated during muscle contraction?

Answer 47

The force per unit area generated by muscle fibres by cross-sectional area

Question 48

What is the difference in force generated between bipennate and parallel arrangement muscles?

Answer 48

- force generated by bipennate can be much larger - bipennate muscles do not expand laterally on contraction - bipennate muscles shorten more slowly then parallel

Question 49

What is the force-velocity relationship for muscle tissue?

Answer 49

- When the resistance (force) is negligible, muscle contracts with maximal velocity - as the load progressively increases, concentric contraction velocity slows to zero at isometric maximum contraction - as the load increases further, the muscle lengthens eccentrically